Complications From Vitamin B Complex

Jessica Bruso

About the Author:

Jessica Bruso

Based in Massachusetts, Jessica Bruso has been writing since 2008. She holds a master of science degree in food policy and applied nutrition and a bachelor of arts degree in international relations, both from Tufts University.

You need the B-complex vitamins, which include naicin, thiamine, riboflavin, biotin, pantothenic acid, folate and vitamins B-6 and B-12, for turning the food you eat into energy. These vitamins also play a role in normal growth and development and proper nerve, muscle and heart function. If you're worried you don't get enough of the B vitamins through your diet, a B-complex vitamin may be beneficial. Taking this supplement may not be right for everyone, however, because the vitamins it contains can have side effects and interact with certain medications.

A pregnant woman is holding supplements in front of her belly.
(Image: mathompl/iStock/Getty Images)

Potential Side Effects

Vitamin B-6 can cause allergic skin reactions, sensitivity to sunlight, loss of appetite, nausea and abdominal pain. Some people experience itching, diarrhea, blood clots or allergic reactions to vitamin B-12. High doses of more than 50 milligrams of niacin can cause flushing and tingling. Thiamine may cause itching. Taking more than 10 milligrams per day of riboflavin could increase your risk of getting eye damage with sun exposure, and cause orange urine, itching, numbness and prickling sensations. High doses of pantothenic acid increase diarrhea risk.

Medication Interactions

All of the B-complex vitamins can interact with the antibiotic tetracycline, so you need to take these pills at separate times. Niacin can interact with nicotine patches, blood thinners, cholesterol medications, blood pressure medications and anti-seizure medications. Riboflavin interferes with a cancer drug called doxorubicin. Folate can interfere with certain chemotherapy medications, including methotrexate, when taken in high amounts.

Toxicity Risk

The B-complex vitamins are water-soluble, so in most cases any excess amounts are excreted in the urine, but a few of these vitamins can still cause adverse effects when taken in large doses. Limit your vitamin B-6 intake to no more than 100 milligrams per day because higher doses may cause nerve damage. Taking more than 200 milligrams per day of vitamin B-6 can cause difficulties with balance and a loss of feeling in your legs. High amounts of niacin could cause stomach ulcers and liver damage.

Other Safety Considerations

Don't take more than the recommended dietary allowance of any B vitamin when pregnant or breastfeeding because this may not be safe for your baby. Getting too much folate may hide the signs of a vitamin B-12 deficiency. People with a hereditary eye disease called Leber's disease shouldn't take vitamin B-12 because it could cause damage, potentially leading to blindness. Don't take niacin if you have stomach ulcers, kidney disease, liver disease, low blood pressure, gout, gallbladder disease or diabetes, and stop taking it at least two weeks before any surgeries you have scheduled. People with hemophilia should avoid pantothenic acid because it can extend bleeding times.